1.需求
- 統計同一所學校的學生人數
2.建表
- 學生表(student)
drop table if exists student;
create table `student` (
`id` int not null auto_increment,
`name` varchar(100),
`age` smallint,
`school` varchar(200) not null,
primary key (`id`)
)engine = innodb default charset = utf8
- 學校人數表(school_num)
drop table if exists school_num;
create table `school_num` (
`school` varchar(200) not null,
`num` int default 0,
primary key (`school`)
)engine = innodb default charset = utf8;
3.觸發器
- add_school_num
-- 新增時更新學校人數
drop trigger if exists add_school_num;
create trigger add_school_num
after insert on `student` for each row
begin
if (new.school in (select school from school_num)) then
update school_num set num = num + 1 where school = new.school;
else
insert into school_num (school, num) values (new.school, 1);
end if;
end;
- delete_school_num
-- 刪除時更新學校人數
drop trigger if exists delete_school_num;
create trigger delete_school_num
after delete on `student` for each row
begin
if (old.school in (select school from school_num)) then
update school_num set num = num -1 where school = old.school;
end if;
end;
- update_school_num
-- 更新是更新學校人數
drop trigger if exists update_school_num;
create trigger update_school_num
after update on `student` for each row
begin
if (new.school in (select school from school_num)) then
update school_num set num = num + 1 where school = new.school;
else
insert into school_num (school, num) values (new.school, 1);
end if;
if (old.school in (select school from school_num)) then
update school_num set num = num - 1 where school = old.school;
end if;
end;
4.測試
4.1 插入資料
4.1.1 執行sql
insert into `student` (name, age, school) values ('張三', 22, '北京大學');
insert into `student` (name, age, school) values ('李四', 25, '北京大學');
insert into `student` (name, age, school) values ('王五', 21, '清華大學');
insert into `student` (name, age, school) values ('趙六', 19, '北郵大學');
4.1.2 student表資料
4.1.3 school_num表資料
4.2 刪除資料
4.2.1 執行sql
delete from `student` where name = '趙六';
4.2.2 student表資料
4.2.3 school_num表資料
4.3 更新資料(更新為已存在的學校)
4.3.1 執行sql
update `student` set school = '北郵大學' where name = '王五';
4.3.2 student表資料
4.3.3 school_num表資料
4.4 更新資料(更新為不存在的學校)
4.4.1 執行sql
update `student` set school = '傳媒大學' where name = '李四'
4.4.2 student表資料
4.4.3 school_num表資料
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